Several million removable complete dentures are fabricated, relined, or rebased every year in the United States and most are made of hard acrylic bases which closely fit the maxillae and palate or the mandibular ridge. This hard denture often pinches or irritates the soft tissues lying against the bone. Disease states which make this problem more severe include excessive alveolar ridge resorption, various mucous membrane diseases, bone irregularities, and psychological aversions to wearing dentures. Soft and resilient denture liners have been used to remedy these problems, but such plasticized acrylics, silicone rubbers, polyurethane rubbers, or polyvinyl chlorides are not successful as permanent materials. Previous work at GSRI, partly funded by NIDR, has led to the development of polyphosphazine fluoroelastomer (PNF) for use as permanent soft denture liner. This rubber was developed by Firestone Tire and Rubber Company and has excellent mechanical properties. The development at GSRI has achieved curing at boiling water temperatures, good bonding to acrylic and both a soft and firm material for use in the denture over the bony anatomy and at the periphery of the denture, respectively. Lab testing and toxicity studies are essentially complete and indicate the need for human trials. A clinical study is proposed via subcontracts to Northwestern University Dental School (Postgraduate Prosthodontic Clinic) and Charity Hospital in New Orleans (Dental Department). In a pilot study, a battery of fabrication, physical property, clinician evaluation and patient-evaluation tests will be conducted to detemine their reliability and validity, using patients now wearing temporary soft liners. The principal study will treat 70 patients at Northwestern and 35 patients at Charity Hospital over 4.5 years by constructing sets of dentures with PNF in one denture and a commercial silicone rubber liner in the other. The patient will act as his/her own control. At three-month (later six-month) intervals, patients will be recalled and evaluated. Data will be statistically tested to control the effects of the different medical centers, and will be directed toward determining whether or not PNF is superior to silicone rubber.